Current transformers are used in AC power supply
systems to sense generator line current and to provide
a current, proportional to the line current, for circuit
protection and control devices.

The current transformer is a ring-type transformer
using a current carrying power lead as a primary (either
the power lead or the ground lead of the AC generator).
The current in the primary induces a current in
the secondary by magnetic induction.

The sides of all current transformers are marked “H1"
and “H2" on the unit base. The transformers must be
installed with the “H1" side toward the generator in
the circuit in order to have proper polarity. The secondary
of the transformer should never be left open
while the system is being operated; to do so could cause dangerously high voltages, and could overheat
the transformer. Therefore, the transformer output connections
should always be connected with a jumper
when the transformer is not being used but is left in
the system.

Transformer Losses

In addition to the power loss caused by imperfect coupling,
transformers are subject to “copper" and “iron"
losses. The resistance of the conductor comprising the
turns of the coil causes copper loss. The iron losses are
of two types called hysteresis loss and eddy current
loss. Hysteresis loss is the electrical energy required to
magnetize the transformer core, first in one direction
and then in the other, in step with the applied alternating
voltage. Eddy current loss is caused by electric
currents (eddy currents) induced in the transformer
core by the varying magnetic fields. To reduce eddy
current losses, cores are made of laminations coated
with an insulation, which reduces the circulation of
induced currents.

Power in Transformers

Since a transformer does not add any electricity to the
circuit but merely changes or transforms the electricity
that already exists in the circuit from one voltage to
another, the total amount of energy in a circuit must
remain the same. If it were possible to construct a
perfect transformer, there would be no loss of power
in it; power would be transferred undiminished from
one voltage to another.

Since power is the product of volts times amperes, an
increase in voltage by the transformer must result in
a decrease in current and vice versa. There cannot be
more power in the secondary side of a transformer than
there is in the primary. The product of amperes times
volts remains the same.

The transmission of power over long distances is
accomplished by using transformers. At the power
source, the voltage is stepped up in order to reduce the
line loss during transmission. At the point of utilization,
the voltage is stepped down, since it is not feasible to
use high voltage to operate motors, lights, or other
electrical appliances.

DC Measuring Instruments

Understanding the functional design and operation of
electrical measuring instruments is very important,
since they are used in repairing, maintaining, and
troubleshooting electrical circuits. The best and most
expensive measuring instrument is of no use unless the technician knows what is being measured and
what each reading indicates. The purpose of the meter
is to measure quantities existing in a circuit. For this
reason, when a meter is connected to a circuit, it must
not change the characteristics of that circuit.

Meters are either self-excited or externally excited.
Those that are self-excited operate from a power source
within the meter. Externally excited meters get their
power source from the circuit that they are connected
to. The most common analog meters in use today are
the voltmeter, ammeter, and ohmmeter. All of which
operate on the principles of electromagnetism. The
fundamental principle behind the operation of the meter
is the interaction between magnetic fields created by
a current gathered from the circuit in some manner.
This interaction is between the magnetic fields of a
permanent magnet and the coils of a rotating magnet.
The greater the current through the coils of the rotating
magnet, the stronger the magnetic field produced.
A stronger field produces greater rotation of the coil.
While some meters can be used for both DC and AC
circuit measurement, only those used as DC instruments
are discussed in this section. The meters used
for AC, or for both AC and DC, are discussed in the
study of AC theory and circuitry.